1. Field of the Invention
This invention relates to methods and apparatus used in modeling and processing a subterranean formation. In particular, the invention relates to using the quantitative interpretation of microseismic data. Also in particular, the invention relates to methods to estimate the b-value, complete frequency-magnitude distribution, and detection limit curves for a formation.
2. Description of the Related Art
Hydraulic fracturing is used to increase hydrocarbon production. During a hydraulic fracture treatment, a fracturing fluid is injected at a pressure exceeding the in-situ stress of the target formation to create a large fracture. In a rock formation that does not contain extensive natural fractures, a single dominant fracture is created in the direction perpendicular to the minimum in-situ stress. The resulting fracture, filled with propping agent carried by the fluid, provides a highly conductive conduit to facilitate the flow of hydrocarbon into the wellbore.
In recent years, microseismic monitoring has been widely used in hydraulic fracture treatments to help determine the dimensions of the created hydraulic fracture. Often, during the hydraulic fracturing process, the natural fractures or faults existing in the formation undergo slippages along the natural fracture planes, triggering a series of small magnitude seismic waves traveling in the formation, called microseismic events, created by the stress increase and fluid filtration in the region surrounding the fracture. These microseismic events can be detected by a string of geophones located in a neighboring well or in one individual well. By processing the detected acoustic wave forms, the epicenter of each microseismic event can be determined. Collectively, the detected event locations form an estimate that encompasses the actual fracture being created. Based on the microseismic data, engineers can estimate the length and height of the hydraulic fracture. Human judgment is often relied upon to exclude isolated or sparse events in this exercise, leading to large uncertainties in the inferred fracture dimensions.
With increasing application of hydraulic fracturing in formations such as fractured shales, microseismic monitoring provides evidence that complex hydraulic fracture networks are created in the highly naturally fractured formation. Manual extraction of the fracture shape from a microseismic cloud is possible, but is quite challenging and highly uncertain. Thus, effective interpretation of microseismic events demands a robust and automated fracture extraction method. The use of the amplitude information to enhance microseismic map interpretation has been introduced. Various fracture complexity indicators may include seismic deformation information. However, a system is needed that relates the amplitude ratio for each observed event to the orientation of the major plane. Further, the system needs to provide a quantitative interpretation with assigned probabilities.